Proposed work for the 1980-1981 grant year involves extension of the studies carried out in the current year. Two distinct areas of research are involved: (A) The kinetics of interaction between adrenal steroid 11Beta-hydroxylase and an activator present in adrenal cytosol. The proposed studies in this area involve: 1) determination of kinetic changes of interaction between these components and stage of pregnancy; 2) determination of the equilibria scheme of interaction between enzyme, substrate, and activator; and 3) elucidation of physicochemical properties of activator. (B) Equilibrium interactions between progesterone and placental progesterone receptors. The proposed studies in this area include: 1) elucidation of association and dissociation kinetics of progesterone-receptor binding and the influence of an endogeneous inhibitor on these processes; 2) the action of the inhibitor on the thermodynamics of ligand-receptor interaction; 3) validation of an exchange assay for placental nuclear receptors: and 4) the influence of the ACTH-adrenal cortex axis on activity of the inhibitor and subsequent alterations in progesterone-receptor interaction.